A recent post mentioned the mill-scale on hot-rolled mild-steel being hard or abrasive, reminding me of some unexpected problems with such steel.

The mill-scale (oxide) is very thin, however, can there be an appreciably thicker layer of hard steel just below it?

In the first instance, two of us were hole-sawing large bolt-holes through a piece of standard channel section. The saw cut most of the way through but really struggled with the last part of the cut, perhaps as much as a sixteenth of an inch.

In the second I was tapping the holes in the baseplate of my Harrison lathe cabinet – a welded steel fabrication – to take hefty great levelling-screws. It was a brute of a task anyway, but the last part, when the tap was breaking though the 5/8 inch plate, was the worst.

In both cases it was the exit side of the work that objected. The channel was a decidedly pre-loved piece that had lost any mill-scale long ago.

So could there be a hard layer in the steel itself, under the oxide? If so, would I expect to find it on both sides or was the similarity above, co-incidental?

Thank you gentleman for the various ideas.

I can't speak for the origins of the channel section, which could have had inclusions. We noticed this problem on at least two holes, but I think we did manage it eventually by turning the piece over and coming in from the other side.

I think Harrison would have bought good-quality steel for making lathe cabinets.

The cabinet base is a single, massive steel plate about 5/8-inch thick. It's occurred to me that as the holes I were trying to tap are close to flame-cut edges and welds, I wonder if steel has small, partially-carburised areas, a sort of case-hardening.

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I managed it but as the holes were a bit under 1.125 BSW tapping-size I had to "pilot tap" them with a metric tap , having carefully drawn the threads to an enlarged scale to assess the number of pitches possible before the mis-match became excessive. By the time I finished I wished I'd simply used a crow-bar and shims!

I did consider that, and it would make sense, though I think it would need more and heavier supports than just those small studs. Though I have used this method for other machines, I decided against it here for two reasons.

First, I was not certain I would keep the lathe in one place as I developed the workshop.

Secondly the floor below the cabinet is completely hidden by the base-plate so I could not guarantee grouting the entire cavity properly, especially since the back of the box is only a few inches from the workshop wall.

As it happens the lathe will probably stay in that location. For a start I have mounted the motor on a sub-frame fitted to a frame supporting the overhead hoist rails, so moving the machine is not just a simple matter of crow-bars and rollers. The workshop floor is reasonably smooth and level concrete but not to machine-tool accuracy, and the cabinet overlaps a rough shuttering-scar a few inches wide, along the walls.

The Harrison manual does refer to levelling screws but my lathe stand had only plain holes, so probably pre-dates that edition of the handbook.

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I have used at work an equivalent to your aircraft-engineering technique. Two specially-profiled aluminium wedge pairs had to hold a rectangular assembly to the narrow inner ends of an elliptical fibre-glass shell in such a way the wedges could be pressed in to a calculated pre-stress. The rear faces of the outer wedges were therefore machined to a close-fitting elliptical arc, but bedded into the shell itself by a similar resin compound allowed to cure under a minimal load before the full insertion. On completion the projecting wedge ends were trimmed back flush and everything stayed permanently in place – and they proved extremely difficult to dismantle safely anyway.

The biggest headache was of the wedges' bearing faces galling, so they had to be coated liberally with anti-seize grease, and usually successfully.